1. Field of the Invention
The present invention relates to a steering angle detection device for detecting the steering angle of a steering wheel.
2. Discussion of Background
Recently, it has been practiced to control the steering force generated by a power steering apparatus in dependence upon vehicle speed and steering wheel rotational angle by utilizing a microcomputer. In a steering angle detection device used for detecting the rotational angle of a steering wheel for this purpose, there is used an encoder which outputs a pulse signal each time a steering shaft is rotated a unit angle, and the pulse signal is input to a microcomputer to be counted. A steering angle detection device of this type is advantageous in respect of required space as well as accuracy.
However, in the above-noted steering angle detection device using an encoder, the absolute steering angle cannot be detected, and the turning-off of an ignition switch causes a steering angle being stored in a memory to be lost. Thus, a relative rotational angle which is calculated taking as an origin the position where the steering wheel was located at a previous stop state is detected when electric power is subsequently supplied, whereby a neutral position (straight running condition) of the steering wheel cannot be discriminated.
As one method of solving this problem, it may be conceived to back up the microcomputer by a constant voltage circuit of a small power consumption lest the turning-off of the ignition switch should cause the content of the memory to be lost. However, this method cannot be a complete solution because no countermeasure can be taken in the event of a battery exhausted.